The present invention relates to a freeze-dried product produced from a frozen product of a material to be freeze-dried, obtained by mixing the material with dry ice, by removing dry ice and frozen moisture (ice) included in the material from the frozen product under sublimation, and to a process and an apparatus for the production thereof.
Methods have heretofore been proposed for producing freeze-dried products which are obtained from frozen products of materials to be freeze-dried frozen with dry ice and are produced by removing dry ice and contained ice therefrom. For example, a process has been proposed in Japanese Patent No. 3005657 C, which comprises mixing the material to be freeze-dried, such as a raw biotic material, drug, food or industrial raw material, with dry ice under compression to form a frozen mass, crushing the frozen mass into disintegrated frozen fragments and drying the frozen fragments under vacuum condition.
By processing the material to be freeze-dried by mixing it with dry ice so as to attain substantially instantaneous freezing of the material while replacing ambient atmosphere by carbon dioxide gas evolved from dry ice under sublimation and while drying the resulting frozen material under vacuum condition, the freeze-dried product of the material, which retains the original quality and properties of the material as such, can be obtained using a simple apparatus with easy operation without suffering from denaturation of the material due to the influences of oxygen, enzymes and heat. The resulting freeze-dried product, which is an inactive matter retaining the quality and properties of the original material, is permissible of being stored, transported and utilized without suffering from denaturation by the action of oxygen, heat etc.
Conventionally employed freeze-drying apparatuses have a construction in which a plurality of heating shelves in a form of a multistage unit are installed in a tightly sealed vessel provided with an exhaustion manifold and a flap closure lid for opening and closing the open end of the vessel. The frozen material to be freeze-dried is placed on each shelf and the shelves holding the frozen material are brought in the internal chamber of the sealed vessel via the open end. The vessel is closed and the tightly sealed chamber retaining the shelves is evacuated to vacuum, whereupon the shelves are heated to effect vacuum drying of the frozen material on the shelves, while the gases and moisture given off under sublimation from the frozen material are sucked out into a cold trap via the exhaustion manifold to condense the condensible ingredients, such as moisture etc., to remove them.
By such a conventional freeze-drying apparatus, a large scale processing of materials is difficult due to restrictions in the amount and the size of the material to be freeze-dried and due to the restricted volume of the interstitial space between the shelves, since many heating shelves are installed with inevitable reduction of total internal free spaces of the chamber. A further problem adds thereto that an irregular heat transfer on heating the material to be freeze-dried may occur due to confinement of the material within a narrow interstitial spaces between the shelves, resulting in occasional denaturation of the material caused by a possible local heat accumulation in the material at the portion near the heating surface, whereby a non-uniform product quality may occur. For example, a freeze-dried product of slices of strawberry or the like may suffer from a problematic phenomenon of adhesion of slices to each other at the mutual contact faces when the slices are held put together one over another, whereby the performance of freeze-drying may considerably be reduced due to reduction of free surfaces of the material. While a rotary pump has heretofore been used for the vacuum pump connected to the exhaust manifold, a higher vacuum in the tightly sealed vessel may difficultly be attained in case the material to be freeze-dried is present as a mixture with dry ice, since a considerable amount of carbon dioxide gas is evolved by sublimation of dry ice, so that efficient freeze-drying will not be realized due to the reduction in the vacuum-drying efficiency.
An object of the present invention is to provide a process for producing a high quality freeze-dried product of a material to be freeze-dried, in which the material mixed and frozen with dry ice to form a frozen product can be deprived of its moisture, while effecting an efficient removal of carbon dioxide gas evolved from dry ice by sublimation in a considerable amount and while excluding any adhesion of individuals of the frozen product to each other by maintaining the frozen product under rotation, whereby the time required for freeze-drying the material can be reduced and a uniform heating of the material can be realized.
Another object of the present invention is to provide an apparatus for producing a high quality freeze-dried product of various materials, in which the material mixed and frozen with dry ice to form a frozen product can be deprived of its moisture, while effecting an efficient removal of carbon dioxide gas evolved from dry ice by sublimation in a considerable amount and while excluding any adhesion of individuals of the frozen product to each other by maintaining the frozen product under rotation, whereby the time required for freeze-drying the material can be reduced and a uniform heating of the material can be realized.
A further object of the present invention is to provide high quality freeze-dried products using the process and apparatus as given above.
The present invention consists in the following process and apparatus for producing a high quality freeze-dried product as well as the freeze-dried product obtained thereby:
(1) A process for producing a freeze-dried product of a material to be freeze-dried, comprising the steps of
placing in a rotary cage installed in a sealed vessel of a freeze-drying apparatus a gas-permeable container containing one or more frozen products of the material to be freeze-dried which are obtained by freezing the material with dry ice,
heating the frozen material by a heating device in the freeze-drying apparatus under vacuum condition and
evacuating the sealed vessel to vacuum so as to cause sublimation of dry ice and frozen moisture included in the material under heating to produce the freeze-dried product.
(2) The process as defined in the above (1), wherein the freeze-drying is performed under variation of the heating condition in accordance with the state of sublimation of dry ice.
(3) The process as defined in the above (1) or (2), wherein the freeze-drying is performed under variation of the evacuating condition in accordance with the state of sublimation of dry ice.
(4) An apparatus for freeze-drying a material to be freeze-dried, comprising
a sealed vessel enclosing a processing chamber, a rotary cage installed in the processing chamber rotatably and served for receiving a gas-permeable container for containing one or more frozen products of the material to be freeze-dried which are obtained by freezing the material with dry ice,
a heating device arranged so as to permit to heat the frozen material to be freeze-dried,
a cold trap with cooling element, connected to the processing chamber and
an evacuating unit for evacuating the processing chamber via the cold trap to a reduced pressure.
(5) The apparatus as defined in the above (4), wherein the heating device is constructed so as to permit variation of the heating condition in accordance with the state of sublimation of dry ice.
(6) The apparatus as defined in the above (4) or (5), wherein the evacuating unit is constructed so as to permit variation of the evacuation condition in accordance with the state of sublimation of dry ice.
(7) A freeze-dried product produced by the process as given in any one of the above (1) to (3).